1. Field of the Invention
This invention relates to a process for the production of Na dichromate using the sodium hydrogen sulfate melt formed in the production of chromic acid from sodium dichromate and sulfuric acid.
2. Background Information
In addition to Na hydrogen sulfate, this melt contains free sulfuric acid, chromic acid, chromium sulfate and an insoluble Na-Cr sulfate complex salt. Their respective proportions in the melt depend upon the reaction conditions used in the chromic acid production process.
The chromium contents are on average from 2.5 to 3.5% CrO.sub.3 and from 0.5 to 1.5% Cr.sub.2 O.sub.3. On account of the high H.sub.2 SO.sub.4 content in the form of hydrogen sulfate and free sulfuric acid and the content of hexavalent chromium, attempts have repeatedly been made to use a solution of the hydrogen sulfate to acidify the monochromate solutions in the production of dichromate. However, the presence of trivalent chromium compounds leads to the formation and precipitation of Cr(III)/Cr(VI) compounds which are difficult to filter and which lead to such considerable production difficulties in the manufacture of dichromate that they are no longer used.
It is known from U.S. Pat. No. 4,045,340 that around 80% of the monochromate solutions are acidified with the waste hydrogen sulfate and that the Cr(III)/Cr(VI) compounds are separated in two stages in a boiling process. After the Cr(III)/Cr(VI) compounds have been filtered off, the filtrates are introduced into the dichromate production process free from trivalent chromium compounds. The Cr(III)/Cr(VI) compounds serve as starting material for the production of Cr(III) salts.
According to U.S. Pat. Nos. 3,715,425 and 3,784,672, a bisulfate solution substantially free from trivalent chromium and suitable for acidification of the monochromate solution is obtained by reaction of the bisulfate solution with persulfate anions and heating to 60.degree. to 120.degree. C. The persulfate anions were prepared by anionic oxidation of bisulfate solution on platinum anodes. However, this process is very expensive.